Tomographic examinations such as computer tomography (CT) or magnetic resonance imaging (MRI) are now common practice as part of clinical imaging. In MRI examinations, magnetic resonance signals are captured from a slice, e.g. from a thin, three-dimensional cuboid. The slice orientation, e.g. the angular alignment of the slice in a three-dimensional space, gives the orientation of the image plane of the cross-sectional image. Whereas in CT, fluoroscopic images are captured along a spiral trajectory and then reconstructed, MRI can be used to produce cross-sectional images directly in any orientation.
MRI image production is done by superimposing two or more gradient fields on a homogeneous main magnetic field. Each gradient field may be produced by a fixed gradient coil. The gradient fields may be oriented along a gradient axis, e.g. a gradient coil can be used to produce a magnetic field that rises or falls linearly and is parallel to these physical gradient axes. These physical gradient axes are mutually orthogonal, e.g., oriented perpendicular to one another, and may be labeled as the x-axis, y-axis and z-axis.
Each gradient coil normally has a limit on a possible gradient strength, e.g. on an amplitude of the gradient field that can be produced by the gradient coil and a limit on a gradient slew rate, e.g. on a rate of rise of the gradient fields.